1. Field of the Invention
The present invention relates to an ultrasound imaging method and a diagnostic ultrasound system capable of producing a contrast echo image by utilizing the nature of echoes that are enhanced due to the strong scattering characteristic of an ultrasound contrast medium, which is injected into a patient body, relative to ultrasonic waves, and of measuring the velocity of the contrast medium using a Doppler technique.
2. Description of the Related Art
In recent years, contrast echo techniques using an ultrasound contrast medium have drawn attention in the field of analysis of myocardial images.
A myocardial contrast echo technique accompanied by intra-arterial injection of an ultrasound contrast medium through the artery has been studied as one of the contrast echo techniques. The technique is used to produce a perfusion image and thus assess a region that is perfused by way of a myocardial blood flow. The myocardial contrast echo technique is such that an ultrasound contrast medium (for example, 5%-diluted human albumin having a foam produced manually or by means of a sonicator) is injected via a catheter indwelled in the aorta. A region perfused by way of a myocardial blood flow is displayed as an enhancement area in a B-mode image owing to the contrast medium. Likewise, a contrast echo technique accompanied by intra-arterial injection has been studied even for examination of an abdominal region in an effort to assess a tumor perfused by the hepatic artery. A diagnostic ultrasound system for general studies and/or a workstation are used as a diagnostic system in which these contrast echo techniques are implemented. Thus, enhancement in a B-mode image may be evaluated through a visual check, or a change in luminance level may be evaluated quantitatively after image data stored in a memory is properly processed within the workstation.
Moreover, recently, development of ultrasound contrast media themselves is in earnest. Ultrasound contrast media enabling assessment of the left heart through intravenous injection have been developed. An ultrasound contrast echo technique using this kind of contrast medium has been put to trial.
The ultrasound contrast media include, for example, "a microbubble which is made by infusing air into an albumin membrane produced when the 5%-diluted human albumin is processed by ultrasonic waves and in which an average diameter of a particle is approximately 4 micrometers" (product name: Albunex injection 5 ml) which is imported and sold by Shionogi Co., and Ltd.
Among the aforesaid known contrast echo techniques, a contrast echo technique accompanied by intra-arterial injection makes it necessary to indwell a catheter in the aorta and is therefore limited to hospitals having a relatively large facility (operating room) enabling the indwelling. Moreover, a large load must be incurred by a patient because this diagnostic procedure is invasive. For these reasons, the scanning technique will presumably not penetrate general clinical sites.
Bubbles of a contrast medium to be injected intra-arterially are relatively large. The bubbles are likely to become stagnant in the cardiac muscle or a capillary bed region. An enhancement may persist for several minutes. Therefore, even when Doppler measurement is performed on these vascular regions, the moving velocity of bubbles does not accurately reflect a blood flow velocity.
By contrast, a contrast echo technique accompanied by intravenous injection is the least invasive and imposes only a small load on a patient. However, a contrast medium reaches the cardiac muscle or any other intended region through the lung. Compared with the contrast echo technique accompanied by intra-arterial injection, the concentration of a contrast medium is low and the degree of enhancement is low. In a region prone to the influence of echoes emanating from surrounding tissues; such as, the cardiac muscle or the peripheral region of the abdomen, it is very hard to observe enhancement attained by a contrast medium. It is the current situation that this technique cannot be employed in assessment of a region perfused by way of a myocardial blood flow using a perfusion image or detection of a blood flow in the parenchyma of the liver.
Moreover, for producing color Doppler images, the influence of motion artifacts stemming from the motions of surrounding tissues or the motions of vascular walls, which are caused by respiration or the like, is so large in examination of, especially, the abdomen, that a produced image does not serve as a color Doppler image.